Modern cyclotrons employ a concept called "sector focusing" to constrain the vertical dimension of the accelerated particle beam within the poles of the cyclotron magnet.
The magnet poles contain at least three wedge-shaped sectors, commonly known as "hills", where the magnetic flux is mostly concentrated. The hills are separated by regions, commonly referred to as "valleys", where the magnet gap is wider. As a consequence of the wider gap the flux density, or field strength, in the valleys is reduced compared to that in the hills.
Vertical focusing of the beam is enhanced by a large ratio of hill field to valley field; the higher the ratio, the stronger are the forces tending to confine the beam close to the median plane. The tighter the confinement, in turn, the smaller the magnet gap may be (in principle) without danger of the beam striking the pole faces in the magnet.
This is important since, for a given amount of flux in the gap, a magnet with a small gap requires less electrical power for excitation than does a magnet with a large gap.
In the limiting case of the "separated sector cyclotron" each hill sector is a complete, separate, stand-alone magnet with its own gap, poles, return/support yoke, and excitation coil. In this implementation the valleys are merely large void spaces containing no magnet steel. Essentially all the magnetic flux is concentrated in the hills and almost none is in the valleys.
In addition to providing tight vertical focusing, the separated-sector configuration allows convenient placement of accelerating electrodes and other apparatus in the large void spaces comprising the valleys.
More recently, superconducting magnet technology has been applied to cyclotrons. In superconducting cyclotron designs, the valleys are also large void spaces in which accelerating electrodes and other apparatus may be conveniently emplaced. The magnet excitation for a superconducting cyclotron is usually provided by a single pair of superconducting magnet coils which encircle the hills and valleys. A common return/support yoke surrounds the excitation coil and magnet poles.
For a given radius of acceleration this configuration affords a much more compact and efficient structure than the separated-sector configuration.
The large hill-to-valley field ratio of the separated-sector cyclotron, combined with the relatively more compact and efficient physical implementation of the superconducting cyclotron, is embodied in the non-superconducting "deep-valley" magnet configuration disclosed in International Patent No. PCT/BE86/00014.
Whereas the "deep valley" cyclotron configuration achieves a high value magnetic field with relatively low excitation, there are inherent inefficiencies in having to utilize two magnet coils, and conventional coil designs have not taken full advantage of the inherent efficiencies of the "deep valley" cyclotron configuration. In this regard, conventional magnet coils are typically wound using insulated hollow-core conductor to allow water-cooling so as to remove heat from the interior of the windings. The conductor packing factor (the ratio of conductor volume to total volume) in coils utilizing such conductor is generally less than 50%, resulting in higher electrical resistance, relatively high power requirements, and more heat to be removed from the windings. Moreover, the hollow-core conductor commonly used for magnet coils is generally available only in short pieces which must be carefully joined and wrapped with insulation to make up the required lengths. The work must be done carefully and checked meticulously to insure leak-free joints of lasting electrical and mechanical integrity. After winding is complete, the coils are generally cured by vacuum potting in epoxy or by vacuum-varnish-impregnation to insure stability and durability. Accordingly, the overall process is lengthy, labor intensive and expensive.
Therefore, it is an object of the present invention to provide a cyclotron which utilizes a single magnet coil to achieve greater energy efficiency.